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2 The con-tap test has the ablty to ndcate damage n a structural element due to a localzed change of stffness. The change n vbraton sgnature may be detected by ear or more precsely by measurement of the dynamc contact force. In ths paper, a con-tap test method for dscrmnatng between measurements made on sound and delamnated structures s present. It has been shown that the characterstcs of contact force hstory durng a tap are changed by the presence of defect beneath the surface of the structure. For structurally radated nose, the sound feld s drectly coupled to the structural moton. Therefore, Impact response analyss should be performed. And the delamnaton model s used to analyze the mpact response analyss. Expermental results for delamnated composte structure are presented and correlatons between smulaton and measured data are shown. It s shown that con-tap test s useful and practcal dagnostc tool for detectng localzed delamnaton n composte lamnates. 1 Introducton The use of lamnated composte structures has many potental applcatons n a varety of engneerng felds. However, they are mostly used n a lamnated form wth relatvely weak nterlamnar nterfaces whch are vulnerable to transverse loads such as mpacts arsng from a fallng mass. The presence of delamnatons can cause sgnfcant degradaton of the structural response characterstcs [1]. The con-tap test s one of the oldest methods of nondestructve testng and t s regularly used for testng lamnated structures. The test requres an operator to tap each pont of the structure to be nspected wth a con, and lsten to the resultng sound radated by structure. When a structure s struck wth a hammer, the characterstcs of the mpact are dependent on the local mpedance of the structure and on the hammer used. Damage such as an adhesve dsband and fatgue damage results n a decrease n structural stffness, and hence a change n the nature of mpact []. Adams showed that t s possble to produce a verson of the con-tap test whch depends on the measurement of the force nput to the test structure durng the tap [3]. There are two knds of con-tap test method. One s based on the mpact force hstores that produced when the structure s struck. And the other s based on sound pressure hstores. The frst one requres the tme hstores of the force nput to the test structure to be captured, and ths can be done va transducer n tappng head so no transducer need be attached to the structure. The second requres the tme hstores of the sound that radated from the structure. The sound based con-tap test requres the Fourer transform of the sound pressure hstores to be computed and compared wth a standard from a sound structure. It s shown that the two knds of con-tap test are useful and practcal dagnostc tool for detectng localzed delamnaton n composte lamnates. In ths paper, an mpact force based non-destructve method s proposed. Ths method s usng the dfference between measured mpact force hstores of healthy structure and delamnated structure. For structurally radated nose, the sound feld s drectly coupled to the structural moton. Therefore, mpact response should be analyzed. Generally, the fnte element method on the mpact response of the lamnate has been known to requre long computaton tme. Shvakumar et al. dd not use the fnte element method to analyze the mpact response of the lamnate but used the sprng-mass model to effcently predct the mpact force hstory [4]. In ther study, the contact energy due to local ndentaton as well as transverse shear energy and bendng energy of plate are consdered, and they reported that the contact energy can be neglected n the mpact by relatvely low velocty foregn object on a flexble or thn plate. However, ther study was restrcted to crcular lamnates wth transversely sotropc materal propertes. Cho proposed the sprng element model usng lnearzed contact law. In ther study, they have shown that the lnearzed contact law approach could be appled to low-velocty mpact response analyss problem wth usng general purpose FEM software [5]. In ths study, to use the sprng-mass model, hammer shaped mpactor s modeled by concentrated mass. And to nvestgate the mpact response on delamnated lamnates, gap elements are used to avod the overlap and penetraton between the upper and lower sub-lamnates at delamnaton regon. The use of three-dmensonal elements to predct the mpact response of delamnated composte structures s nconvenent because of a qute number of elements necessary to obtan numercal solutons. In ths study, -D fnte elements are used n delamnated area. In ths study, from the mpact response analyss results, sound pressure s computed. Impact response analyss Fg.1 shows the FEM model for mpact response analyss usng general purpose FEM software. The mass of mpactor s lumped at the end of the sprng mass, and the other end of sprng element s attached to lamnate at mpacted locaton. After FEM analyss we can extract the compressve force hstory actng at the sprng. In present study, MSC/NASTRAN was used as general-purpose FEM software. In FEM modelng 4-node plate element was used and transent dynamc analyss was performed wth the ntal condton, whch s that ntal speed of the lumped mass was loaded as mpact velocty. Fg. 1 Sprng - mass model usng general-purpose FEM software.1 Smplfed sprng mass model Fg. shows the FEM model for mpact response analyss for hammer shaped mpactor. The hammer shaped mpactor 46

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